The field of the invention relates to engine control systems, including air/fuel and ignition control systems.
It is known to alternately deliver fuel to the engine rich and then lean while the converter is below a desired temperature. Retarding ignition timing to more rapidly heat engine exhaust gases and the catalytic converter coupled to the exhaust gases is also known. An example of such a system is shown in U.S. Pat. No. 5,211,011.
The inventors herein have recognized numerous problems with the above approaches. One problem is excessive modulation may result in engine misfires and diminished modulation may result in ineffective air/fuel control. Another problem is that in attempting to provide average air/fuel operation slightly lean of stoichiometry, actual air/fuel operation may be leaner than desired causing rough engine operation and increased emissions.